DISCUSSION OF BACKGROUND
A light reflecting and heat dissipating material which is capable of reflecting light and dissipating heat transmitted thereto from a layer in contact with or adjacent to the material to cool the layer is applicable to an optical information recording medium.
For instance, in a phase-change type optical recording method, there are utilized changes in an optical constant of a recording material used in a recording layer of a phase-change type optical recording material, which are caused to occur during the phase transition of the recording material by the application of heat thereto.
More specifically, for example, in the phase-change type optical recording method, recording is carried out as follows:
A chalcogen compound is used as the above-mentioned recording material for the recording layer of the phase-change type optical recording medium. A portion of the recording layer is exposed to a light with an energy which is equal to or more than the energy corresponding to the energy band gap of the chalcogen compound, so that the light is absorbed in the exposed portion. The light absorbed in the exposed portion is converted to heat, so that the temperature of the exposed portion of the recording layer is raised by the heat to fuse the exposed portion. The fused portion is then rapidly cooled so as to assume an amorphous state. The amorphous portion is used as a pit for recording. In order to erase the pit, the amorphous portion is exposed to a light with a power lower than that of the above-mentioned light employed for recording so that the amorphous portion is crystallized.
Thus, the phase-change type optical recording method basically utilizes phase changes between a crystalline state and an amorphous state of a recording material in accordance with an intensity modulation of an incident light to the recording material. Therefore, in the phase-change type optical recording method, it is important how to record information and erase recorded information with high reliability by low-energy light application (i.e., with high recording sensitivity). The improvement of the recording sensitivity depends upon how effectively the light applied to the recording layer is absorbed in the recording layer, and also upon how effectively the heat generated in the recording layer is utilized.
As a specific method for carrying out the above-mentioned improvement, it can be proposed to decrease the thickness of the recording layer, thereby decreasing the heat capacity of the recording layer and reducing the light energy required for recording and erasing information. Also, it can be proposed to most effectively use the light applied to the recording layer by providing a reflection layer on the recording layer so as to cause the light which has passed through the recording layer to perform multiple reflection within the recording layer.
For instance, in Japanese Patent Publication 61-18262, there is proposed a method of the above-mentioned type, in which a chalcogen compound is used in a recording layer, and a metal such as Al, Ag, or Rh is used in a reflection layer. In this method, the light reflection layer has high thermal conductivity, and therefore can rapidly cool the recording medium by conducting heat from the recording layer through the light reflection layer. Thus, the recording medium can easily assume an amorphous state for recording information.
However, when a metal with high thermal conductivity such as Al, Au or Cu is employed alone in the light reflection layer for the recording medium, before the temperature of a recording portion reaches a sufficiently high temperature for information recording (that is, a temperature necessary for fusing the recording portion) the recording medium dissipates heat through the light reflection layer so quickly that the recording sensitivity is lowered, and stable recording cannot be carried out.
There is another proposal for the improvement of the recording sensitivity, in which a metal such as Ti, Cr, Mn or Ni, or a semiconductor material such as Bi, with relatively low thermal conductivity, is used in a light reflection layer as disclosed in Japanese Laid-Open Patent Application 63-58637. In this case, however, unless the thermal conductivity of the material used in the light reflection layer is adequately controlled, there are the risks that (1) the thermal conductivity of the light reflection layer becomes too small to obtain a sufficient cooling effect for causing the recording medium to assume an amorphous state, so that stable information recording cannot be performed, and (2) the reflectivity of the light reflection layer is so decreased that the light reflection layer fails to perform the light reflecting function.